1. Technical Field
The disclosed embodiments relate generally to varactor circuits for fine tuning and/or to switchable capacitor circuits for coarse tuning in Voltage-Controlled Oscillators (VCOs).
2. Background Information
FIG. 1 (Prior Art) is a symbol of a Voltage Controlled Oscillator (VCO) 1. FIG. 2 (Prior Art) is a more detailed diagram of the VCO 1 of FIG. 1. VCO 1 includes an LC tank 2. The natural resonant frequency of the LC tank 2 largely determines the oscillating frequency of the differential VCO output signal output by the VCO 1 onto differential output conductors 3 and 4. In the illustrated example, the inductance of the LC tank 2 is provided by inductor 5. The capacitance of the LC tank 2 is provided by a combination of capacitance elements including a varactor circuit 6 and a coarse tune capacitor bank 7. The coarse tune capacitor bank 7 is made up of a number of parallel-connected switchable capacitor circuits. Reference numeral 25 identifies one switchable capacitor circuit. By changing the digital input signals EN[1:N] and ENB[1:N] supplied via input conductors 14 and 15 to the coarse tune capacitor bank 7, capacitances provided by individual ones of the switchable capacitor circuits can be selectively switched into and out of the LC tank 2. Switching out switchable capacitor circuits decreases the overall capacitance of the LC tank 2, thereby increasing VCO 1 oscillating frequency. Switching capacitor element into the LC tank 2 increases the overall capacitance of the LC tank 2, thereby decreasing VCO 1 oscillating frequency.
Fine tuning of the capacitance of the LC tank 2 is accomplished by adjusting a fine tune analog input signal VTUNE on input lead 8. The fine tune analog input signal VTUNE affects a voltage across varactors 9 and 10 and this fine tunes the capacitance of the LC tank 2. How VTUNE affects the varactors 9 and 10 is determined by how the varactors are biased. In one example, increasing VTUNE decreases the voltage across the main varactors, thereby decreasing the capacitance provided by the main varactors 9 and 10, and thereby increasing the VCO 1 oscillating frequency. Conversely, decreasing VTUNE results in increasing the voltage across the main varactors, thereby increasing the capacitance provided by the main varactors 9 and 10, thereby decreasing the VCO 1 oscillating frequency.
It is desired that the oscillating frequency of the VCO 1 be a function of VTUNE and of the coarse tune digital input signals EN[1:N] and ENB[1:N], and be substantially independent of temperature. If, however, there were no temperature compensation circuitry provided, then for a given fixed VTUNE voltage the oscillating frequency of the VCO 1 would be seen to decrease with increasing temperature due to the capacitances of the main varactors 9 and 10 increasing with increasing temperature. To compensate for this, the VCO 1 of FIG. 2 includes temperature compensation varactor circuitry in the form of varactors 11 and 12. These varactors 11 and 12 are coupled in parallel with the main varactors 9 and 10 of LC tank 2 of the VCO 1. A temperature compensation voltage signal PTAT is applied via input lead 13 to these varactors 11 and 12. The voltage signal PTAT increases with increasing temperature. As temperature increases, the voltage PTAT increases. The increasing PTAT decreases the voltage across the temperature compensation varactors 11 and 12, thereby reducing their capacitances. The decreasing of the capacitances of the temperature compensation varactors 11 and 12 as temperature increases counteracts the affects of how the increasing temperature causes the capacitances of the main varactors 9 and 10 to increase.
As illustrated in FIG. 2, the varactor circuit 6 is AC coupled to inductor 5. The varactor circuit is therefore DC biased via bias resistors 16 and 17. There are noise sources, such as resistor noise introduced by resistors 16 and 17 and such as power supply noise present in voltage VBIAS at voltage supply conductor 18. The power supply noise manifests itself as common mode noise across the varactors. Noise from these sources affects the varactors and increases VCO phase noise in an undesirable way. An improved VCO architecture is desired.
In addition, the VCO 1 may be required to operate over a wide frequency range, for example from 3.0 GHz to 5.0 GHz. To accommodate such a wide frequency range, a capacity to switch in and out the capacitances provided by the switchable capacitor circuits of the coarse tune capacitor bank 7 is required. In some applications, there is a stringent phase noise requirement imposed on the VCO 1 at some operating frequencies. In order to meet this stringent phase noise requirement, the VCO 1 is made to operate with a large AC voltage swing across the LC tank 2 between nodes 19 and 20. The AC voltage swing may be, for example, 2.5 volts peak-to-peak when the VCO 1 is operating at 4.0 GHz. The RF transceiver integrated circuits of which the VCOs are a part of are now often made using thin gate oxide 65 nm or 45 nm MOS semiconductor fabrication processes. Due to the thin gate oxides of the transistors made using such small geometry processes, voltages across the transistors must be maintained below about 1.5 volts in order to prevent the large voltages from causing breakdown and otherwise damaging the transistors. If a simple thin oxide transistor of such a semiconductor process were used for switch 21, then the transistor may break down and fail.
FIG. 3 (Prior Art) is a diagram that illustrates one method of avoiding this problem. A special transistor 22 having a thicker oxide gate dielectric is used. In order to achieve the same performance as is achieved using the thin gate oxide transistor 21 of FIG. 2, however, the thick gate oxide transistor 22 of FIG. 3 is made to be larger. Providing the larger transistor increases parasitics, including parasitic capacitances. It is desired to be able to reduce the overall capacitance of the LC tank 2 when many of the capacitors of switchable capacitor circuits of the coarse tune capacitor bank 7 are switched off so that the VCO 1 can oscillate at a high frequency. The parasitic capacitances of all of the many transistors of the switchable capacitor circuits, however, may combine to be such a large capacitance that the overall LC tank capacitance cannot be reduced as low as required for high frequency VCO 1 operation.
FIG. 4 (Prior Art) is a diagram that illustrates a second method of avoiding the problem with thin gate oxide transistor 21 of FIG. 2 breaking down. In this second method, two thin gate oxide transistors 23 and 24 are provided in series as illustrated. Each of these transistors sees only half of the AC voltage swing between nodes 19 and 20, and therefore can survive the high voltage swing of the VCO 1 without suffering breakdown. To achieve the same performance as with the thin gate oxide transistor 21 of FIG. 2, however, the on resistance through the two transistors should be low. Due to there being two transistors in series rather than one, the sizes of the transistors 23 and 24 are doubled to achieve an adequately low on resistance. This increase in transistor size again increases parasitic capacitances. The parasitic capacitances of all the transistors of all the switchable capacitor circuits of the coarse tune capacitor bank 7 combine such that the lower limit of the capacitance of the LC tank 2 is too high for high frequency VCO 1 operation. Moreover, the series-connected transistors of the topology of FIG. 4 are observed to pick up or otherwise to introduce an increased amount of unwanted noise. The exact mechanism by which this noise is introduced is not fully understood, but an improved VCO architecture is desired.